1. Field of the Invention
The invention pertains generally to the manufacture of glass sheet, and, more particularly to glass sheet formed from an overflow process.
2. Description of Related Art
This invention relates to the manufacture of glass sheet that is currently used for the production of TFT/LCD display devices that are widely used for computers now and are expected to be used for television sets in the future.
The glass that is used for these semiconductor powered display applications must have very high surface quality to allow the successful application of semiconductor type material. Sheet glass made using the apparatus of U.S. Pat. No. 3,338,696, incorporated herein by reference, makes the highest quality glass as formed and does not require post-processing. The patent teaches a manufacturing process termed: xe2x80x9cthe overflow processxe2x80x9d. The key feature of the overflow process is that the glass moves through the glass forming equipment and the untouched, xe2x80x9cvirgin glassxe2x80x9d, overflows and becomes the outside surface of the glass. Glass made using other processes requires grinding and/or polishing and thus does not have as fine a surface finish.
The teachings of U.S. Pat. No. 3,338,696 are still the state of the art as practiced today, however, the apparatus has limitations.
A major drawback of the present overflow process is the limited range of production rates that can be accommodated by a given apparatus. Simple mathematics teaches that the mathematical product of glass flow, glass viscosity, and the tangent of the tilt angle must be a constant to make glass of uniform thickness for a given apparatus. In practice the angle of tilt has practical limitations and the simple mathematical assumptions fail because of non-linearity, thus the limited range of production rate.
There is also no adjustment provision for curvature or other non-uniformity in the thickness of the glass sheet. During a manufacturing campaign the forming apparatus is subject to changes in shape caused by material erosion and by thermal creep of the material from which the apparatus is constructed. This produces non-uniform sheet thickness for which there is no corrective adjustment.
In practice, glass forming processes are intended to be run for years if possible. Stopping the line may take a month and re-starting another month. The whole system is run at extreme temperature, so the materials used are under constant stress and often wear or need to be adjusted. Stopping the production to make these adjustments or repairs is a highly undesirable option.
The apparatus of xe2x80x9cThe Overflow Processxe2x80x9d makes excellent glass during stable operating conditions, but it recovers from transient conditions very slowly. This is because the flow of glass through the forming apparatus is quite non-uniform. The glass that flows into the apparatus and forms the inflow end of the sheet has a different time history than the glass forming the far end of the sheet. Because the glass flow is from one end to the other, on the order of 10 percent of the glass on the far end of the sheet is at least one hour older than the equivalent glass on the near end. An additional time differential is caused by the rectangular cross-section of the trough which has regions of glass where the flow velocity is very slow.
Another drawback of the apparatus shown in U.S. Pat. No. 3,338,696 is that its physical size becomes larger as production rate is increased. Apparatus durability often requires that parts of the device are made from or coated with refractory metal, i.e. platinum. Thus increasing the size of the system substantially raises the cost of the manufacturing apparatus. The weir overflow process apparatus becomes large as production rates increase because the flow of glass in the trough, channeling glass to the overflow weirs, relies on gravitational flow. As the glass flow rate increases, either the trough must be larger, the slope of the weirs steeper, or the glass viscosity lower to accommodate this increased glass flow. Therefore, the range of production rates that can be accommodated by a given size apparatus is limited.
Another drawback is that the apparatus can only make glass sheet of a fixed width.
The present invention includes design features that can be used alone or in combination to facilitate faster, more uniform flow of glass through the apparatus and allows the thickness of the sheet to be adjusted.
An overflow device is provided at the far end of the trough and is used in conjunction with tilting of the apparatus to expand the range of glass flow rate and glass viscosity for which this invention will produce satisfactory product. The overflow device may be used in conjunction with tilting of the apparatus, changes in glass flow rate, and changes in glass viscosity (temperature) to regulate the thickness profile of the sheet. In addition, the forming apparatus can be made with non-linear weirs to provide a greater range of flow rates.
The glass flow in the inflow pipe can be modulated to provide more uniform time dependent flow for forming the sheet. Similarly, a contoured trough cross-section is taught which can minimize the quiescent flow regions.
The forming apparatus can include an orifice on top of the trough and glass can be moved through the apparatus using pressure. The orifice is narrow at the inlet end and wider at the far end to accommodate for the loss in static pressure of the glass as it flows to that end.
A forming apparatus can be provided with additional orifices on the bottom or sides to allow greater variability in sheet thickness. Effectively, an additional orifice adds glass to the middle of the formed sheet.
Irregularities in the thickness of the formed glass sheet may be corrected by selective heating of the glass in the trough, heating the weirs or orifice. This requires selective heating of the glass flowing down the outside of the apparatus to restore temperature uniformity at the bottom of the apparatus where the sheet is formed.
A flow control plug can be inserted into the trough, such that flow dynamics can be altered during hot operation by insertion, removal or position adjustment of the flow control plug.